Kofi Annan, former secretary-general of the United Nations and Nobel Peace Prize winner, shares a moment on stage with former Canadian governor-general Adrienne Clarkson in Ottawa on May 23, 2013. Mr. Annan delivered a lecture at an annual event by the Global Centre for Pluralism. (COLE BURSTON FOR THE GLOBE AND MAIL)

Kofi Annan, former secretary-general of the United Nations and Nobel Peace Prize winner, shares a moment on stage with former Canadian governor-general Adrienne Clarkson in Ottawa on May 23, 2013. Mr. Annan delivered a lecture at an annual event by the Global Centre for Pluralism.(COLE BURSTON FOR THE GLOBE AND MAIL)

Former United Nations secretary-general Kofi Annan is warning that Syria’s increasingly sectarian civil war threatens to “explode” beyond its borders and that the remaining hope rests on forging the united international pressure for a political agreement that has eluded the world so far.

Mr. Annan spent five months last year as the UN and Arab League envoy trying to negotiate a peace in Syria, but he resigned in August, saying that a lack of unified international support had hobbled his mission.

Now, a year later, Syria is not imploding the way Libya did when rebels drove out Moammar Gadhafi, he noted.

“If anything, it’s likely to explode, and explode beyond its own borders,” Mr. Annan said in Ottawa as he delivered a lecture at the Global Centre for Pluralism. “The involvement of regional interests, proxy wars and the paralysis of international decision-making – more specifically the Security Council – has created a truly poisonous mix that threatens to spill over into neighbouring countries.”

Some would argue that the violence has already begun to spill into communities in Lebanon and Iraq, Mr. Annan said, and the risk of fighting between Arab Sunnis and Kurdish forces could drag in Turkey, and it has increased tensions between Iran and Gulf nations and deepened divisions between Sunni Shia.

He said he still believes the violence can be subdued through negotiation and mediation, pointing to a planned peace conference next month backed by both the United States and Russia, as a hope – as long as the world unites behind it. That will require the involvement of not just the U.S. and Russia, but other influential countries on both sides of the conflict between the regime of President Bashar al-Assad and oppositon rebels, like Iran on the regime’s side, and Qatar, Saudi Arabia and Turkey, which support opposition groups.

Mr. Annan, who has previously said he believes military intervention now will probably do more harm, told an audience member that there isn’t really a “Plan B” to united international pressure: “When it comes to an issue like Syria, which is a hard, tough issue, we need to be coldly realistic and accept that the parties which have been fighting for two, 2 1/2 years, will only bend to the will of the powerful countries.”

The former UN chief, delivering a lecture on bridging ethnic and religious divides at the centre, an organization devoted to that purpose created by the Aga Khan and the Canadian government, contrasted diplomacy in Syria with his involvement in mediation after bloody violence erupted over Kenya’s 2007 elections: Mediators there were strongly backed by the region and the world and brought opponents to the table for a process that led to a new constitution.

But he said the world missed an opportunity in Syria a year ago, when his peace plan, including a transitional government, was endorsed by countries on both sides – but the backing was “lots of lip service.” And he blamed countries on both sides, including the United States and Russia, who “talked past each other.”

The U.S. and the West were able to lay easy blame on Russia and China for blocking resolutions in the UN Security Council, he said. But all had agreed on a plan to create transitional government with full power, and that should have been “halfway home,” he said. But the West insisted that Mr. al-Assad had to leave office first, while the Russians warned that would bring chaos. The two countries should have held talks on establishing a transitional government in a few months to ease Mr. al-Assad out. “Rather than pointing fingers at each other and calling each other names,” he said. “That’s not leadership.”